Exhaust line of a diesel engine and desulfation method

ABSTRACT

A method for desulfating an NOx trap of an exhaust line of a diesel engine, the method to extract during the operation of the engine at least one portion of the fixed sulphur compounds in the NOx trap. The method produces a reformate via a reformer device from the fuel of the engine, preheat the NOx trap via injection of the reformate in the flow of exhaust gases, in an injection point located downstream of the engine is upstream of the NOx trap, until a predetermined desulfation temperature is reached in the NOx trap, then bypasses the flow of exhaust gases from a bypass point of the exhaust line located upstream of the injection point, and continues the injection of reformate in the NOx trap to produce the desulfation, the richness of the mixture supplying the diesel engine not being affected in the method.

The present invention relates to an exhaust line of a diesel engine and to a method of desulfating a NOx trap of this line, the method being intended to extract, during the operation of the engine, at least a portion of the sulfur compounds fixed in said NOx trap.

Many exhausts of motor vehicle diesel engines can use catalytic devices, commonly termed NOx traps, capable of trapping the nitrogen oxides NOx in the presence of an excess of oxygen in the exhaust gases, that is to say under richness conditions of below 1, in order to reduce the level of nitrogen oxide in the exhaust gases emanating from the vehicles. However, sulfur compounds, in particular SOx, arising from the fuel and the lubricating oil, are present in the exhaust gases and are preferentially adsorbed on the catalytic sites of the NOx trap, blocking these sites, with the result that a periodic regeneration of the NOx trap catalyst, that is to say a purge of the SOx, generally referred to as desulfation, is necessary for the NOx trap to resume its activity of storing NOx.

This regeneration consists in raising the temperature of the NOx trap above a minimum desulfation temperature, of between 400 and 900 degrees, often greater than 550°, and in creating therein richness conditions greater than 1 by injecting reducing gases into the NOx trap.

Document WO 99/00 588 describes a regeneration consisting in raising the temperature of the NOx trap by operating the engine with a richer mixture than during normal operation. This method has the drawback of increasing the fuel consumption, of having a negative impact on the operation of the engine and the vehicle driving comfort and can result in a dilution of the engine oil. Documents EP 1 055 806 and EP 1 106 798 also describe regeneration systems based on the post-injection of fuel, with the same drawbacks. Document US 2005/0000 210 describes a system based on the injection of diesel fuel or of reformate after the temperature of the NOx trap has been raised by modifying the richness of the mixture supplying the engine. Documents FR 2 838 770 and DE 199 39 807 describe systems based on the injection of hydrogen upstream of the NOx trap, which therefore require a specific hydrogen reservoir.

The object of the present invention is to provide a desulfation method which is not intrusive with respect to the engine while minimizing the overconsumption of fuel.

This object is achieved by a method of desulfating a NOx trap of an exhaust line of a diesel engine, this method being intended to extract, during the operation of said engine, at least a portion of the sulfur compounds fixed in said NOx trap, comprising the following steps:

-   -   producing a reformate via a reformer device from the fuel of the         engine,     -   preheating the NOx trap by injecting the reformate into the flow         of the exhaust gases at an injection point situated downstream         of the engine and upstream of the NOx trap, until a         predetermined desulfation temperature is reached in said NOx         trap, and then     -   branching off the flow of the exhaust gases from a branch point         on the exhaust line situated upstream of the injection point and         continuing the injection of reformate into the NOx trap to         achieve said desulfation, the richness of the mixture supplying         the diesel engine not being affected in this method by         performing the above steps.

Implementing the method according to the invention therefore requires an exhaust line comprising a bypass from the branch point upstream of the NOx trap and means, such as a 3-way valve, for branching the exhaust gases via the bypass, together with a reformer, that is to say a device which makes it possible to convert hydrocarbons, such as the fuel of the diesel engine, into a mixture of reducing gases, termed reformate, composed mainly of CO, H2 and N2.

The reformate can be produced by a technique selected from partial oxidation reforming (POX, Partial OXidation), steam reforming and autothermal reforming (ATR, Auto Thermal Reforming), which are techniques known per se.

The duration of the branching step can be predetermined, in particular this duration can be between 10 seconds and 20 minutes.

The duration of the preheating step can also be predetermined.

The duration of at least one of the preheating and/or branching steps can also be determined by a temperature measurement, which is representative of the temperature of the catalytic sites of the NOx trap.

At the end of the branching step, the exhaust gases can be redirected into the NOx trap and the production of reformate can be halted.

According to another embodiment, the production of reformate can be continued while several preheating and branching steps alternate in cycles.

With the method according to the invention it is simultaneously possible to regenerate a particulate filter, commonly termed PF, arranged in the exhaust line downstream of the NOx trap, the flow rate of the reformate being adjusted in this embodiment to reach a predetermined regeneration temperature downstream of said particulate filter, and the exhaust gases branched during the step of desulfating the NOx trap being reintroduced into the exhaust line at a reinjection point situated downstream of the NOx trap and upstream of the particulate filter.

This method can be applied to the regeneration of an exhaust line comprising 2 NOx traps mounted in parallel, a particulate filter mounted downstream of said NOx traps, and a valve system for distributing the flow of the exhaust gases between the two NOx traps, by using a valve system for distributing the reformate leaving the reformer between two injection points arranged in parallel upstream of the first and the second NOx traps, respectively. Each of the two NOx traps can be alternately in a preheating phase and in a desulfating phase during the regeneration of the particulate filter.

In such an exhaust line, purges of NOx from one of the NOx traps can be carried out while the other NOx trap is in a desulfation phase by momentarily inverting the valve systems.

Other particular features and advantages of the invention will become apparent to a person skilled in the art from the description below of three embodiments and from the accompanying figures, in which

FIG. 1 is a schematic representation of a first embodiment of an exhaust line,

FIG. 2 is a schematic representation of a second embodiment of an exhaust line, and

FIG. 3 is a schematic representation of a third embodiment of an exhaust line.

FIG. 1 shows an exhaust line comprising a NOx trap 6 and a reformer 4, the flow of reformate, depicted by dotted arrows, being injected into the exhaust line at an injection point 5. The flow of the exhaust gases 1 emanating from the engine is depicted by solid-line arrows on the left of FIG. 1; these gases can be branched toward a bypass 2 by a valve system 3.

During the preheating step, the reformate is injected into the flow of the exhaust gases which pass through the NOx trap 6. Since these exhaust gases contain oxygen, the latter oxidizes the H2 and CO components of the reformate in contact with a catalyst (noble metal) contained in the NOx trap. This reaction is highly exothermic and the heat thus released allows the NOx trap to be heated to the temperature required for desulfation. The flow rate of the reformate is controlled by means of a control loop in order to achieve the desired temperature range.

When the desulfation temperature, of between 400° C. and 900° C., for example of around 650°, is reached, while continuing the injection of reformate into the NOx trap 6, the valve 3 is activated to branch the exhaust gases 1 toward the bypass 2. Under these conditions, the NOx trap 6 is traversed by a flow of reformate whose richness is equal to that of the air/fuel mixture which supplies the reformer 4. By way of example, in the case of a diesel POx, the optimum richness can be 2.9. Under these conditions, the sulfur present in the NOx trap is released in the form of compounds such as H₂S, COS and SO₂. This desulfation step has a duration which can last typically from 10 seconds to several minutes, for example 20 minutes.

According to a first variant embodiment, after a desulfation step the reformer is deactivated, and the valve system 3 returns to its initial position such that the exhaust gases pass once more into the NOx trap.

According to a second variant embodiment, the reformate is continuously injected into the NOx trap during a succession of several steps. While injecting the reformate, the valve system is regularly actuated between the two positions such that the flow of gas passing through the NOx trap alternates between lean phases and rich phases. During the lean phases, the exhaust gases pass into the NOx trap and the reformate reacts with the oxygen of these gases at the catalytic surface of the NOx trap (platinum or palladium), the exothermic oxidation reaction making it possible to maintain the NOx trap at temperature. During the rich phases, the exhaust gases are diverted via the bypass 2 and the reformate passes alone into the NOx trap such that the latter is decontaminated of the accumulated sulfur. Typically, the duration of the lean phase can last from 20 seconds to several minutes and the duration of the rich phase from 10 seconds to several minutes. The durations of each of the phases are adjusted to suit the catalytic formulation of the NOx trap and its thermal behaviour. One of the main advantages of this alternating rich/lean cycling compared with a continuous bypassing of the exhaust gases is that of better control over the temperature within the NOx trap during the desulfation. It is thus possible to avoid premature ageing of the NOx trap.

The desulfation of a NOx trap as described above can be advantageously coupled to the regeneration of a particulate filter. FIG. 2 shows an exhaust line comprising the same elements 1-6 as those in FIG. 1. Downstream of the NOx trap 6 is mounted a catalyzed particulate filter 8. The exhaust gases which pass into the bypass 2 are fed via a branch pipe 7 to a reinjection point 10 downstream of the NOx trap 6 and upstream of the particulate filter 8. The temperature in the particulate filter 8 is measured using a temperature sensor 9 placed slightly downstream of the filter 8. The value of this temperature is sent to the computer 11 of the reformer 4, which also controls the valve system 3.

The regeneration of the particulate filter 8 requires a temperature of around 600° C. to initiate the combustion of the soot trapped by the filter. In a first step, the valve system 3 for diverting the exhaust gases into the bypass 2 is not activated, with the result that the exhaust gases pass into the NOx trap 6. Reformate is injected at 5, this reformate being oxidized by the oxygen present in the exhaust gases upon contact with the catalytic coating of the NOx trap 6. This exothermic oxidation of the reformate makes it possible both to heat the NOx trap 6 with the aim of desulfating it and to heat the particulate filter 8 by the exhaust gases downstream of the NOx trap. The flow rate of the reformate is adjusted in order to achieve at the sensor 9 a regeneration temperature, typically of around 600° C., which is sufficient to allow the oxidation of the soot stored in the filter 8. When this regeneration temperature is reached, the NOx trap has also reached a sufficient temperature for the desulfation.

In a second step, when the regeneration temperature of the particulate filter 8 is reached, the valve system 3 is activated and the exhaust gases are diverted from the NOx trap and pass into the branch pipe 7. The injection of reformate is maintained such that the NOx trap 6 is subjected to a flow rich in reducing compounds and is decontaminated of the accumulated sulfur by releasing mainly H₂S and COS. The duration of this step can be greater than 10 minutes.

During the desulfation, a large portion, or even majority portion, of the reformate does not react in the NOx trap 6, exits therefrom, mixes with the exhaust gas coming from the branch pipe 7, passes into the particulate filter 8, and reacts with the oxygen contained in the exhaust gas upon contact with the catalytic coating of the particulate filter 8. This exothermic reaction makes it possible to maintain the particulate filter at its regeneration temperature for the entire duration of the desulfation of the NOx trap 6.

For a complete regeneration of the particulate filter 8, the duration of the injection is typically around 20 minutes. If the duration required for the desulfation is much less than the duration required for the regeneration of the particulate filter, the bypass can be deactivated once the desulfation has finished. The injection of reformate is then maintained to allow the regeneration of the particulate filter 8 to continue, the reformate being oxidized by the exhaust gases,

-   -   either on the catalytic surface of the NOx trap if the bypass         has been deactivated,     -   or directly on the catalytic surface of the particulate filter         if the bypass is maintained,         with the result that the particulate filter 8 is maintained at         regeneration temperature. The advantage provided by the second         option (bypass maintained) is to minimize the overconsumption of         fuel supplying the reformer. Specifically, the action of         simultaneously oxidizing the reformate on the particulate filter         and regenerating its soot makes it possible to make immediate         use of the energy released by the exothermic oxidation reaction         of the reformate and thus to minimize the heat losses which         occur between the NOx trap and the particulate filter when         proceeding according to the first option. By proceeding thus, it         is possible to optimize the fuel penalty due to supplying the         reformer with fuel.

The odor of H₂S could cause an unpleasant smell in the immediate environment of the vehicle during the desulfation. However, since H₂S and COS are oxidized in the particulate filter 8 by the exhaust gases, this problem is solved.

The regeneration of the particulate filter can typically take place every 500 km traveled by the vehicle and the desulfation of the NOx trap takes place simultaneously.

The method according to the invention can be applied to an exhaust line comprising two NOx traps mounted in parallel. Such an exhaust line is illustrated schematically in FIG. 3. In this exhaust line, the reformate can be injected upstream of each of the two NOx traps 6 and 6 b using a valve system 12 capable of distributing the reformate leaving the reformer 4 between the two injection points 5 and 5 b. The valve system 3 can switch between the following positions:

-   -   position 1: all the exhaust gases pass into the NOx trap 6     -   position 2: all the exhaust gases pass into the NOx trap 6 b     -   position 3: the exhaust gases are distributed half and half         between the NOx traps 6 and 6 b.

The regeneration takes place as follows:

In a first step, the valve system 3 is in position 1 such that all the exhaust gases pass into the NOx trap 6. The reformate is injected at 5 and oxidized by the oxygen of the exhaust gases on the catalytic surface of the NOx trap 6. This exothermic reaction serves to heat both the NOx trap 6 to its desulfation temperature and the particulate filter 8 to its regeneration temperature. The flow rate of reformate is adjusted in order to achieve, at the sensor 9 downstream of the particulate filter 8, a sufficient temperature to allow the oxidation of the soot in the filter, typically 600° C. When this temperature is reached, the valve system 3 passes to position 2 so as to divert the exhaust gases into the NOx trap 6 b and from there, via the pipe 7 b, into the particulate filter 8.

After the switching of the valve system 3, the injection of the reformate into the NOx trap 6 is maintained, and the latter, at desulfation temperature, is decontaminated of the accumulated sulfur by releasing mainly H₂S and COS. To ensure complete desulfation of the NOx trap 6, this position of the system is maintained for a duration of around 10 minutes or more.

During this phase of desulfating the NOx trap 6, the NOx which are present in the exhaust gases accumulate in the NOx trap 6 b. In order to prevent the NOx trap 6 b from becoming saturated with NOx, purges of NOx from the NOx trap 6 b can be performed: the valve system 3 passes momentarily to position 1 and reformate is injected into the NOx trap 6 b.

During the desulfation, a major portion of the reformate leaves the NOx traps and is oxidized on the catalytic surface of the particulate filter 8, with the result that a filter is maintained at a sufficient temperature for oxidizing the soot during the entire regeneration. Once the regeneration of the particulate filter, which can take around 20 minutes, has finished, the calculator 11 of the reformer 4 cuts the injection and the valve system passes to a position 3 so as to distribute the flow of the exhaust gases into the two NOx traps and allow the NOx to be treated.

During a regeneration operation of the particulate filter, which typically takes place every 500 km traveled by the vehicle, it is possible to desulfate successively each of the two NOx traps by inverting the positions of the valve system 12 and 3, or alternatively to desulfate only one of the NOx traps and desulfate the other during the next regeneration operation of the particulate filter. 

1-14. (canceled)
 15. A method of desulfating a NOx trap of an exhaust line of a diesel engine, the method to extract, during operation of engine, at least a portion of sulfur compounds fixed in the NOx trap, the method comprising: producing a reformate via a reformer device from fuel of the engine; preheating the NOx trap by injecting the reformate into a flow of the exhaust gases at an injection point situated downstream of the engine and upstream of the NOx trap, until a predetermined desulfation temperature is reached in the NOx trap; and then branching off the flow of the exhaust gases from a branch point on the exhaust line situated upstream of the injection point, and continuing the injection of reformate into the NOx trap to achieve the desulfation; and richness of the mixture supplying the diesel engine is not affected by performing the method.
 16. The method as claimed in claim 15, wherein a duration of the branching is predetermined, the duration of the branching being between 10 seconds and 20 minutes.
 17. The method as claimed in claim 15, wherein the duration of the preheating is predetermined.
 18. The method as claimed in claim 15, wherein duration of at least one of the preheating and/or branching is determined by a temperature measurement.
 19. The method as claimed in claim 15, wherein, at the end of the branching, the exhaust gases are redirected into the NOx trap and production of reformate is stopped.
 20. The method as claimed in claim 15, wherein a plurality of preheating and branching operations alternate in cycles, and the production of reformate is continued during the cycles.
 21. The method as claimed in claim 15, wherein the reformate is produced by a technique selected from partial oxidation reforming, steam reforming, or autothermal reforming.
 22. The method as claimed in claim 15, for simultaneously regenerating a particulate filter placed in the exhaust line downstream of the NOx trap, wherein, during the preheating the NOx trap, the flow rate of reformate is adjusted to reach a predetermined regeneration temperature downstream of the particulate filter, and during the desulfating the NOx trap, the exhaust gases are reintroduced into the exhaust line at a reinjection point situated downstream of the NOx trap and upstream of the particulate filter.
 23. A use of a method as claimed in claim 22 in the regeneration of an exhaust line comprising two NOx traps mounted in parallel, a particulate filter mounted downstream of the two NOx traps, and a valve system for distributing flow of the exhaust gases between the two NOx traps, wherein use of a valve system for distributing the reformate leaving the reformer between two injection points arranged in parallel upstream of the first and the second NOx traps, respectively.
 24. The use as claimed in claim 23, wherein, during the regeneration of the particulate filter, each of the two NOx traps is alternately in a preheating phase and in a desulfation phase.
 25. The use as claimed in claim 23, wherein NOx purges of each NOx trap are carried out while the other NOx trap is in a desulfation phase by momentarily inverting the valve systems.
 26. An exhaust line of diesel engines comprising: a NOx trap; a particulate filter arranged downstream of the NOx trap; and a valve device upstream of the NOx trap and a branch pipe allowing the exhaust gases to bypass the NOx trap and to be reinjected upstream of the particulate filter, by presence of a reformer and by presence of a computer capable of controlling a method as claimed in claim
 22. 27. An exhaust line of diesel engines for implementing a method as claimed in claim 23, comprising: two NOx traps mounted in parallel, a particulate filter mounted downstream of the two NOx traps, and a valve system for distributing the flow of the exhaust gases between the two NOx traps; a reformer, of a valve system for distributing the reformate leaving the reformer between two injection points arranged in parallel upstream of the first and the second NOx traps, respectively; and a computer implementing the method.
 28. A method of regenerating a particulate filter of a diesel engine exhaust line as claimed in claim 26, comprising: producing a reformate via the reformer from fuel of the engine and injecting the reformate into the exhaust line downstream of the valve device; branching the flow of the exhaust gases into the branch pipe and reinjecting the gases into the particulate filter; and oxidizing the reformate via the exhaust gases at the active surface of the particulate filter. 